Wet Flue Gas Denitrification With Fe(?)EDTA Accompanying With Regeneration

Nitrogen oxide(NOx)in flue gas is one of the main air pollutants.The solubility value of NO accounting for about 95% of the NOx is so low that the removal of NOx seems to be more difficult.Because of the chemical property of Fe(?)EDTA that can directly combine with NO,denitration with Fe(?)EDTA solution has become an important method to control NOx emission.However,Fe(?)-EDTA can be easily oxidized to Fe(?)-EDTA by O2 in flue gas,losing the ability to bind NO,and causing a higher cost.In order to solve this key issue,urea and metal powders including iron and zinc were employed for the combination agent of Fe(?)-EDTA.The interactions between urea,metal powders and the two products of denitration with Fe(?)EDTA(Fe(?)EDTA and Fe(?)EDTA-NO)were respectively investigated,and the experiments of NO removal using Fe(?)-EDTA combined with urea,Fe powder,and Zn powder were respectively conducted.The details are illustrated as follows:A mixed absorbent containing both(NH2)2CO and Fe(?)EDTA was employed for NO removal.The interactions between urea and Fe(?)EDTA,Fe(?)EDTA,Fe(?)EDTA-NO were respectively investigated.The results indicated that Fe(?)EDTA-NO can be reduced by urea into Fe(?)EDTA,and the oxidation of Fe(?)EDTA can be restrained by urea.Then,the effects of different influencing factors such as urea and Fe(?)EDTA concentrations,temperature,inlet oxygen concentration,and pH value on NO removal were investigated.Results showed that the NO removal efficiency decreased with oxygen concentration increasing,the temperature increasing,and Fe(?)EDTA concentration decreasing.The optimum concentration of urea was 5%-7.5%,and the best pH value was 7.0-9.0.Finally,in order to study the effect of SO32-on NO removal,adding 0.2 mol L-1 Na2SO3 to the mixed absorbent containing 0.02 mol L-1 Fe(?)EDTA and 5 wt% urea was employed for NO removal at 25 oC,pH 7.0,and 7%(v/v)O2.The result showed that SO32-had a positive effect on NO removal and NO removal efficiency could maintain more than 78% for 80 min.Iron powder was proposed as a reductant to reduce Fe(?)EDTA under aerobic condition at room temperature.The effects of different parameters including the initial Fe(?)EDTA concentration,O2 concentration,the molar ratio of Fe to Fe(?)EDTA,and pH value on the reduction rate were evaluated.It can be found that the variation of Fe(?)could be divided into two stages: the Fe(?)increasing stage and Fe(?)decreasing stage.During the Fe(?)increasing stage,O2 could strongly decrease the rate of Fe(?)EDTA reduction.The reduction rate increased as pH decreased.Increasing the molar ratio of Fe to Fe(?)EDTA could raise Fe(?)EDTA conversion.Fe(?)concentration rose as the initial Fe(?)EDTA concentration increased.During the Fe(?)decreasing stage,because of O2,Fe(?)concentrations all began to decrease and the ?-FeOOH precipitate was formed.Finally,the kinetic models of the reduction of Fe(?)EDTA by Fe powder under anaerobic and aerobic condition were respectively obtained.Iron powder was chosen to reduce the Fe(?)EDTA-NO under anaerobic condition.The stoichiometry of Fe(?)EDTA-NO reduction by Fe was derived.Then the effects of Fe(?)EDTA-NO concentration,pH value,and temperature on Fe(?)EDTA-NO reduction were respectively investigated and the kinetics were calculated.The results showed that the reduction rate decreased with the increase of pH value as well as the decrease of temperature.Fe(?)EDTA-NO reduction by Fe powder was a first-order reaction with respect to Fe(?)EDTA-NO.Besides,the energy of activation(Ea)and the entropy of activation(?S?)of the Fe(?)EDTA-NO reduction by Fe powder were calculated to be 23.229 kJ mol-1 and-215.251 J K-1 mol-1,respectively.Finally,the simultaneous reduction of Fe(?)EDTA and Fe(?)EDTA-NO by Fe powder was evaluated.The results indicated that Fe(?)EDTA-NO reduction was rarely restricted by Fe(?)EDTA,and Fe(?)EDTA-NO had a negative impact on the reduction of Fe(?)EDTA.In addition,compared with the rate of Fe(?)EDTA reduction,the reduction rate of Fe(?)EDTA-NO was slower,so Fe(?)EDTA-NO reduction is key during Fe(?)EDTA regeneration.To solve the issue of the slow rate of Fe(?)EDTA-NO reduction,Zn powder was employed as a reductant to reduce Fe(?)EDTA-NO.The stoichiometry and kinetics of Fe(?)EDTA-NO reduction by Zn powder were investigated with respect to Zn powder concentration,pH value,and temperature.The effect of the particle size of the Zn powder on Fe(?)EDTA-NO reduction was also investigated.The experimental results indicated that Fe(?)EDTA-NO reduction rate increased with the decrease of pH value and elevated temperature,and small particle size is beneficial to Fe(?)EDTA-NO reduction.Fe(?)EDTA-NO reduction with Zn powder showed a pseudo-second-order reaction concerning Fe(?)EDTA-NO and the reaction order with regard to hydrogen ions is about 0.14.In addition,the energy of activation(Ea)and the entropy of activation(?S?)of the Fe(?)EDTA-NO reduction with Zn powder were calculated to be 19.853 kJ mol-1 and-135.557 J K-1 mol-1,respectively.Finally,the simultaneous reduction of Fe(?)EDTA and Fe(?)EDTA-NO with Zn powder was investigated.Results showed that Zn powder had better performance on simultaneous Fe(?)EDTA and Fe(?)EDTA-NO reduction as compared to the earlier related researches.These fundamental researches can offer valuable guidance for industrial denitrification using mixed Fe(?)EDTA and Zn powder system.Finally,the experiments of NO removal with Fe(?)EDTA accompanied with the regeneration of Fe(?)EDTA by metal powders in PRBs were conducted.The results showed that the NO removal efficiency could maintain above 90% for 90 min and maintain above 80% for 130 min along with regeneration by Fe powder in PRBs.When it comes to Zn powder,the NO removal efficiency could maintain above 90% for 15 h.The permeable reactive barriers(PRBs)integrated the absorption system presented a good performance on regeneration of Fe(?)EDTA during NO removal.